Salt baths, method of maintaining salt baths neutral, and additive therefor



Patented Sept. 9, 1958 SALT BATES, WTHUD U15 MAMTAINING SALT BATHS NEUTRAL, AND ADDHWE THEREIFQR Arternas l. Holden, Milford, Mich.

No Drawing. Application July 14-, 1955 Serial No. 522,176

9 Claims. (Cl. 252-731) out fluorides of these metals, are commonly used in industry for the heating of metals, as for instance during their heat treatment. s It is well known that, after a period of use, initially neutral baths become alkaline and thus decarburizing to steel due to breakdown of the salts at elevated temperature.

It has been common in the industry to rectify neutral salt baths by adding thereto rectifiers containing either silica, titanium dioxide, silicon carbide, borax, or boric acid. These rectifiers react with the salts to form residual sludges which are basically ceramic in nature. The metal oxides react with these ceramic materials and are separated from the salt bath and enter into a water-insoluble sludge.

Where borax or boric acid is used as a rectifier, either singly or in combination, these components combine with the alkali and metal oxides to form sludges of a more or less glassy type. The action of any of the previously mentioned rectifiers is not sustained but very temporary in character; and, in addition, when rectifiers containing boric acid are added to a molten salt bath, a viscous liquid is formed on the top surface of the bath which persists for a period of from 5 to 20 minutes after addition of the rectifier. The salt bath cannot then be used until total destruction of the boric acid and assimilation of the viscous liquid. Moreover, it is necessary from time to time to remove the residual sludges which, if permitted to remain in the bath, tend to increase ,viscosity of the bath and are water-insoluble so as to be difiicult to wash from the heat treated parts.

I have found that when a composition termed magnesium boron fluoride and consisting essentially of a complex salt of magnesium fluoride, boric oxide, and water and having the approximate formula is added to neutral salt baths composed of salts of the alkali metals and alkaline earth metals, with or without smaller amounts of fluorides of these metals, in amounts between 0.25% and 4% by weight of the total bath, this material is readily assimilated into the bath at a range of temperatures between 840 F. and 2400 F. Magnesium boron fluoride may be added to the bath either initially or after the bath has become contaminated by normal breakdown of the salts from use and will reduce and hold the water insolubles, acid insolubles, and metal oxides to a minimum and maintain the bath in neutral condition.

It is believed that this composition operates through its slow reaction under the effect of elevated temperatures, such as those employed in the above-mentioned baths. The reaction appears to have the effect of releasing compounds Within the bath which react with the metal oxides and with breakdown products of the salts and prevent attack on or decarburization of metals in the bath. Another reaction is believed to take place between the metal oxides and boric anhydride, released by the composition, resulting in insoluble metal pyroborates.

In a bath of the type above described containing the additive of the present invention, the electrodes of the direct-resistance type of salt bath furnace employing immersed electrodes have a much longer life and the life of the pot of a ceramic pot electrode furnace is increased substantially.

Magnesium boron fluoride, which is currently manufactured by the American Agricultural Chemical Company, may be made by mixing magnesium fluoride and orthoboric acid in the solid state in the ratio of four moles of magnesium fluoride to one mole of orthoboric acid and heating the mixture at atmospheric pressure to about C. The reaction produces a complex salt having the approximate formula 4MgF B 0 ZH O Percent by weight Magnesium 25 /z-27 Boron 5.7-5.9 Fluorine 3 33 6 Loss on ignition This calculates to:

Percent by weight MgF 67-70 B 0 18-19 Water (preferably) 9-12 Other (lime, silica, sulfate, fluoride, iron, oxide,

alumina) Trace to about 2 A presently used composition of the material is approximately as follows:

Percent by weight Magnesium fluoride 69.5 Boric oxide 18.5 Water (combined and free) 10.0 Other (lime, silica, sulfate, fluoride, iron, oxide,

alumina) 2 The two percent of miscellaneous impurities present in the ore (such as sellaite) do not appear to have any effect, beneficial or otherwise, either in the manufacture of magnesium boron fluoride or its use as an additive for salt baths.

The following examples of molten salt baths within the scope of the invention are given by way of illustration. It is to be understood that the invention is not to be limited to these specific examples, either as it relates to the relative amount of the ingredients in the bath or to the basic composition of the bath to which the magnesium boron fluoride is added, the scope of the invention being defined by the claims appended hereto.

ition to make up for losses due to on fixtures and parts being treated.

.-NEUTRAL HIGH HEAT BATH omposition:

Percent by weight ed preheat baths in the heat steels, the bath being, operated at F. to 2300 F. This bath was of either of the examples other than addition of bath 0 the initial compos drag-out of the bath Percent by weight 5 EXAMPLE 2 49 T1118 bath had the following initial c 25 25 Barium chloride- 1 Magnesium boron fluoride grating range of salt bath was COD hfl above i with the previously mention gnesium boron fluoride treatfmnt of W 2 breakdown of the salts temperatures 2200 BATH This bath had the following initial composition:

F. When used at slig due to slow EXAMPLE NO. l.NEUTRAL PREHEATING Barium chlorid Potassium chloride Sodium chloride Magnesium boron fluoride This bath is suitable for use at an op 1200 F. to l700 melting point, A percent of ma is all that is required 1. m m m s a a 11 O m 1 Pni mauho S a k x P Er m ta 0t n N H X ho eflE S1 mhh w m bb m h e O t he m t b e 1w o u e mmm m S an fiS fl 0 0 PM m m m nm om ut rOSe S a f 0 m Q I'm 0 e cf W vt w w m wo H d m 1 i m m m mm I d 0 wflhm mmm Sh a e TbbU 2 5 0 1 2 d a flnen wm P fi am b hbf m rame m Pm P hm a Tn M BM H .1 aee. mwmmFm t S a b fou td o e a ud w ,m Pnfi TVI mO a .mh W H .m m Thi S W mm a aFO M w om04m nu muoa dun .I 7 1 n a U a m 1 t a r n .1 eWow .Pm mm hF O GPR E .mt u 0 d S t 0 m e h e m m aw m ma mh a ame P ntr opo 0 3 06 & V. bww m.mw

. e in g 0 3 n v a 1 mm a a 0. o t 3 3V. 3 3 m 08 00 0m 0 0 P 0 0 0 0 g 0 0 V M M M L a m m u u u u u n u in 7 a 3" 5 i n a 1 udm D J 0 D J M O 0 0 O 0 0 0 O 0 0? w w m m a m m m m a a a a a a a a a nu m u m m m u n m m n L 2 1 mm 6 e 0 0 0 0 W W 0 M r O 0 0 0 0 O 0 U 0 mm ow g g g g g g E E i a M w w W W w W W .ltlil] .Illilli iilil ii lulkill llli) Water Insol. Percent c examples of salt bath composi- EXAMPLE NO. 3.--HARDENING BATH m nQQQQQQQQQQQHWLLLZLQQQQQQQ LLLLZLQQUQQQQMOOOOllllO-Z e N MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMQMMMMMOMMMMMOMMMM AP mDu mDuAPAJTAPAPAPAPAPAPAPAPARAPAPAPAPAFAPAPAFAPAPAFAPAP w r dr ivsortwwsanata???isnei itaaaatgitg 444aa4344afijfi fieja ja .4 23. .443 1111111111111111111111111111111111122111122111122 iiirltllilillillll m m m m m m m n u u u a n u n u m m m m n u a n m a u n n B L u n n a a 1 1 a x X l a m m m m m m m m m n H n n n n W P m P P m P P m period of twenty days in the preheating of high-speed The following specifi steel. During this time samples were taken at intervals tions have also been used successfully: and one gram of each sample was titrated with lO/N hydrochloric acid with methyl red as an indicator. The

EXAMPLE NO. 4.-HARDENIN G BATH (Operating Range 1250 F. to 1650 F.)

Potassium chloride 44 Sodium chloride 45 Sodium fluoride 6 Barium chloride 5 Magnesium boron fluoride 1 EXAMPLE NO. 5 .HARDENIN G BATH (Operating Range 1450 F. to 2000 F.)

Barium chloride 77 Calcium chloride 16.5 Sodium fluoride 5.5 Magnesium boron fluoride 1 The use of magnesium boron fluoride as above described in high-temperature salt baths heated with immersed electrodes of Inconel and type 446 chrome electrode metal appears to greatly prolong the electrode life. It has been observed that a bath containing magnesium boron fluoride will tend to reduce chrome oxide to metallic chromium on the surface of the electrodes so as to build up an outer layer of high chromium concentration, which in the case of a 2" X 4" chrome electrode bar formed a high chromium layer of /s" or more after two months operation. While Inconel electrodes formerly had a life expectancy of five weeks when used with neutral chloride salt baths at operating temperatures of 2000 F. to 2300 F., in such baths incorporating magnesium boron fluoride a total life of five to seven months of the same electrodes can be expected. It would appear that in each instance the chromium is reduced and forms a high-chrome outer layer so as to protect the nickel or iron content of the metal from attack by the salt bath.

The usual refractory ceramic pots, including those made of mullite and silica-aluminum refractories, are eroded by the baths due largely to abrasion from metal oxides and other inert compounds of rectifiers. sium boron fluoride does not react with the refractory to decrease pot life, but actually enhances life of the refractory due to reduction of oxide content and less sludge.

In accordance with the invention, marked advantages are realized when magnesium boron fluoride is employed in molten baths consisting of chlorides of one or more of the alkali and alkaline earth metals and with other fluorides at temperatures below 2000 F. where scale is present on the parts being processed. It is apparent from tests that other fluorides are helpful, in amounts of 1% to 20%, i. e., sodium, potassium, barium, calcium, or chromium fluoride, depending on temperature and the process for which the ultimate compound is to be used. Magnesium boron fluoride has been found useful with all chloride-fluoride salt baths.

Magnesium boron fluoride also may be added in amounts from to 4% by weight to neutral baths consisting essentially of hydroxides or carbonates of the alkali and alkaline earth metals and to baths consisting essentially of mixtures of such hydroxides, carbonates,

and chlorides, or of chlorides and hydroxides, chlorides and carbonates, or hydroxides and carbonates, all of the alkali and alkaline earth metals, with or without fluorides of the alkali and alkaline earth metals present. Magnesium boron fluoride in the percentage range above stated 35 2,620,310

Magnehas been added to the salt bath consisting of an eutectic mixture (about 50%50%) of potassium hydroxide and sodium hydroxide with beneficial results in the annealing of gold at a temperature of 1000 F. After quenching in water, the gold when subsequently heated to red heat over open flame showed no tarnish, whereas, under such conditions, a definite tarnish was evident when similarly treated in such a bath in which magnesium boron fluoride was not present.

I claim:

l. A non-aqueous salt bath composition for the heat treatment of metals at temperatures in excess of 840 F., consisting essentially of a mixture of alkali metal and alkaline earth metal halides which normally break down in use to form metal oxides and hydroxides, and from to 4% by weight of a salt having substantially the formula B203 2H20.

2. A non-aqueous salt bath composition for the heat treatment of metals at temperatures in excess of 840 F., consisting essentially of a mixture of salts selected from the group consisting of alkali and alkaline earth chlorides, up to 16% by weight of a fluoride selected from the alkali and alkaline earth fluorides, and from 54 to 4% by weight of a salt having the formula where X can vary from 1% to 2%.

3. A non-aqueous salt bath composition for the heat treatment of metals at temperatures in excess of 840 F., consisting essentially of at least two salts selected from the alkali and alkaline earth halides, and from A to 4% by weight of a salt having the formula where X can vary from 1% to 2%.

4. A bath as defined in claim 2, in which at least by weight of the bath is composed of a mixture of at least two chlorides selected from the alkali and alkaline earth chlorides.

5. A bath as defined in claim 3, in which at least two salts are chlorides.

6. A bath as defined in claim 3, in which at least two salts are potassium chloride and sodium chloride.

7. The method of stabilizing a non-aqueous metal heat treating salt bath containing alkali and alkaline earth halides tending to break down in use at temperatures in excess of 840 F. to form the corresponding oxides and hydroxides, comprising adding to the bath a salt having theformula 4MgF -B O -XH O, where X can vary from 1% to 2 the addition being in such quantities as to maintain the bath substantially free of the breakdown products.

8. The method of claim 7, in which the salt is added in an amount equivalent to from 4% to 4% of the bath by weight.

9. The method of claim 7, in which the salt bath consists essentially of at least one chloride selected from the group consisting of alkali and alkaline earth chlorides.

References Cited in the file of this patent UNITED STATES PATENTS Newell June 6, 1950 Albrecht Dec. 2, 1952 

1. A NON-AQUEOUS SALT BATH COMPOSITION FOR THE HEAT TREATMENT OF METALS AT TEMPERATURES IN EXCESS OF 840*F., CONSISTING ESSENTIALLY OF A MIXTURE OF ALKALI METAL AND ALKALINE EARTH HALIDES WHICH NORMALLY BREAK DOWN IN USE TO FORM METAL OXIDES AND HYDROXIDES, AND FROM 1/4% TO 4% BY WEIGHT OF A SALT HAVING SUBSTANTIALLY THE FORMULA 4MGF2.B2O3.2H2O. 